JavaScript Array Concat Not Working. Why

Javascript Array Concat not working. Why?

The concat method doesn't change the original array, you need to reassign it.

if ( ref instanceof Array )
   this.refs = this.refs.concat( ref );
else
   this.refs.push( ref );

concat does not join JavaScript arrays together?

.concat creates a new Array. You need to overwrite the old one.

scriptUrls = scriptUrls.concat(urls);

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Or if you want to keep the original scriptUrls Array, you can .push() the values in.

scriptUrls.push.apply(scriptUrls, urls);

This uses .apply() to convert urls into individual arguments passed to .push(). This way the content of urls is added to scriptUrls as individual items.

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Also, note that .concat() flattens the Array. If you wanted an Array of Arrays, then you'd use scriptUrls.push(urls).

Why can't I concatenate these two arrays?

From documentation - The concat() method is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array.

One way to use concat would be to do the following

state.building.panoramas = state.building.panoramas.concat(panoramas)

Or you can simply

[].push.apply(state.building.panoramas, panoramas);

Javascript concat not working as expected - not merging

Declare array before for loop, set fixed.innerHtml after and read about javascript scope.

var fixed = document.getElementById('fixed');
var sections = document.getElementsByClassName('section');
var arr = [];
for (i = 0; i < sections.length; i++) {  arr = arr.concat(sections[i]);}
fixed.innerHTML = arr;

<div class="fixed" id="fixed"></div><div class="section"></div><div class="section"></div><div class="section"></div><div class="section"></div><div class="section"></div>

JavaScript concat not working as expected, care to elaborate?

As per documentation of Array.prototype.concat:

Returns a new array comprised of this array joined with other array(s)
and/or value(s).

Which means it does not modify the object it is applied to.

Change to:

allCities = allCities.concat(cities[c]);

javascript recursive function concat not working

Array.prototype.concat()

The concat() method is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array.

So those lines do nothing:

if("sp" in val) values.concat(getValues(val.sp))
else if("it" in val) values.concat(getValues(val.it))

You need to write:

if("sp" in val) values = values.concat(getValues(val.sp))
else if("it" in val) values = values.concat(getValues(val.it))

And you should not use map if you don't use it's result. Use forEach instead.

Why is Array.push() throwing an error but Array.concat() does not?

.concat returns a new array with both the current items and the new ones where .push modifies the current array instead of returning a new one.

If that array was initially loaded from vuex it results in an unwanted mutation of the store.

Edit:

In the store

mutations: {
    addFile(state, file) {
      state.ItemData[0].Files.push(file);
    },
}

In the component

NewFiles.forEach(file => store.commit("addFile", file));

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